1. Field of the Invention
The present invention relates to a strut-type suspension device for an automobile and in particular relates to a construction thereof for improving straight traveling stability during driving in a straight path.
2. Description of the Related Art
As shown in FIG. 4, in a conventional strut-type suspension device, an insulator 3 holding an elastic rubber element 2 in its interior is provided in a vehicle body 1. The upper end of a strut 7 is held in the elastic rubber element 2 within this insulator 3 and an upper seat 5 is arranged below the insulator 3, with a bearing 4 interposed therebetween. A lower seat 8 is fixed to the strut 7 and a compression coil spring 6 is held between this upper seat 5 and the lower seat 8. In this case, the central axis of the bearing coincides with the axis of the strut. 9 is the axis of a kingpin.
Japanese Utility Model Publication No. S61-93305A discloses improvements in a strut-type suspension device arranged such that torsion of the spring generated when the spring is displaced is released, by mounting a self-aligning bearing on an upper seat or a lower seat.
Also, in Laid-open Japanese Patent Publication No. 9-300932A, a rotational moment is generated in opposite directions in the right side suspension means and the left side suspension means. One of the beneficial effects of this is that the lowering of straight traveling stability is avoided.
FIG. 5 is a diagram of the operation of a conventional strut-type suspension device. In this case, the bearing is arranged so as to intersect the strut axis (the double-dotted chain line) at right angles. The upper seat 5 rotates about the strut axis 12 (=bearing axis 13) and the lower seat 8 rotates about the kingpin axis 9 (single-dotted chain line). Since the upper seat 5 and the lower seat 8 have respectively different axes of rotation, in general, a moment is generated about the kingpin, impairing the straight traveling stability.
FIG. 6 shows an imaginary plane at the upper seat of FIG. 5. The origin of the X and Y co-ordinates is centered on the strut axis 12, the forward direction of the vehicle body being X and the width direction of the vehicle body being Y. The point where the line of action of the load 14 intersects the upper seat or lower seat is called a point of action of the load 15. The relationship between the rotation angle θ about the kingpin axis and the strain energy stored in the spring when the kingpin axis is shifted in the X direction will now be examined wherein the point of action of the load is fixed. Throughout the specification, the rotation angle about the kingpin axis means the angle by which the upper or lower sheet rotates about the kingpin axis, unless otherwise specified.
FIG. 7 shows the relationship between the rotation angle θ and the strain energy in the prior art device when the bearing axis and the strut axis coincide. Taking the amount of eccentricity of the point of action of the load from the Y-axis as a parameter, it can be seen from the Figure that the strain energy of the spring varies with the rotation angle θ. Rotation of both the upper and lower seats therefore takes place in the direction such as to reduce the strain energy. For example if the amount of eccentricity is −10.3 mm, rotation takes place with a rotation angle θ in the negative direction and if the amount of eccentricity is 18 mm, rotation takes place with a rotation angle θ in the positive direction. This appears as a rotation moment about the kingpin.
It can also be seen from FIG. 7 that the direction of the rotation moment generated changes depending on whether the amount of eccentricity in the X direction is positive or negative. Consequently, if the amount of eccentricity is zero, even if rotation takes place, the strain energy is constant and a rotation moment is not generated.